I. Field
The following description relates generally to wireless communications, and more particularly to delivering sequenced service data units (SDU) to wireless communication devices.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as, for example, voice, data, and so on. Typical wireless communication systems may be multiple-access systems capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power, . . . ). Examples of such multiple-access systems may include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, and the like. Additionally, the systems can conform to specifications such as third generation partnership project (3GPP), 3GPP long term evolution (LTE), ultra mobile broadband (UMB), etc.
Generally, wireless multiple-access communication systems may simultaneously support communication for multiple mobile devices. Each mobile device may communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from base stations to mobile devices, and the reverse link (or uplink) refers to the communication link from mobile devices to base stations. Further, communications between mobile devices and base stations may be established via single-input single-output (SISO) systems, multiple-input single-output (MISO) systems, multiple-input multiple-output (MIMO) systems, and so forth. In addition, mobile devices can communicate with other mobile devices (and/or base stations with other base stations) in peer-to-peer wireless network configurations.
MIMO systems commonly employ multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. The antennas can relate to both base stations and mobile devices, in one example, allowing bi-directional communication between the devices on the wireless network. Mobile devices can travel about one or more wireless network service areas. To facilitate service access while moving, mobile devices can handover communications from one base station to another when moving within a specific range of the target base station. For the purpose of this description a handover may refer to a hand over of a communication from a base station to another base station as well as a hand over from and to the same base station. Further, the hand over may be initiated by the network or by the mobile terminal. The handover may also occur in order to support mobility of users in the wireless system, or to provide balancing of load, or to facilitate various reconfigurations of the connection or to facilitate handling of unforeseeable error cases. Additionally, mobile devices can employ re-transmission schemes, such as hybrid automatic repeat request (HARQ), to replicate delivery of data ensuring a higher probability of successful receipt.
In this regard, data packets can be delivered from a mobile device to a base station out-of-order, as a HARQ process re-transmits unsuccessful packets while it also continues to transmit new packets. Data packets can be re-ordered by an appropriate protocol following potentially unordered delivery. However, when communications are handed over from a source to a target base station, the sequencing of the packets by the appropriate protocol is facilitated at the target base station by providing information to the target base station about the sequence number of the first packet to expect.